Integrated circuit designs and fabrication are well known in the art. More generally integrated circuits have been classified as one of two types: memory or logic (including processors). Memory circuits, such as volatile or non-volatile memories, store information. Logic circuits, such as AND gates, OR gates, NOR gates, NAND gates, inverters, XOR gates, XNOR gates, Multiplexers, processor, and look-up tables, process signals.
In certain applications, it is desired to combine memory circuits with logic circuits in a device. Thus, one type of device is a field programmable logic device, wherein connection of logic circuits may be altered based upon states stored in the associated memories. Typically, the memories are field programmable, i.e. the state of the memory can be programmed or erased, after the device has been manufactured (or is in the field). One class of memory is non-volatile memory, where the state of the memory is retained after the power supply is disconnected from the device.
Another potential application of a device having both memory circuits and logic circuits, wherein the states stored in the memory circuits is used to alter the connection of the logic circuits is where a manufacturer has designed a plurality of products all having the same basic design but with different specifications. Rather than manufacturing (and maintaining inventory of) the plurality of different devices, the manufacturer could design a basic device with different logic circuits that are activated depending upon the specification desired. In this manner inventory management is greatly improved.
The prior art discloses a non-volatile memory cell of the type having a floating gate for storage of charges. The memory cell has a first region and a spaced apart second region, with each region being of a first conductivity type with a channel region of the second conductivity type therebetween. The floating gate is spaced apart from a first portion of the channel region and is adjacent to the first region. A coupling gate is over the floating gate and is capacitively coupled to the floating gate. A control gate is adjacent to and spaced apart from the floating gate and the coupling gate. The control gate is spaced apart from another portion of the channel region and has a portion adjacent to the second region. An erase gate is over the first region and is spaced apart therefrom and spaced apart from the floating gate and the coupling gate. See U.S. Pat. No. 6,747,310, whose disclosure is incorporated by reference in its entirety.
In the prior art it is also known to use two non-volatile memory cells connected in series to form a storage element to control a logic circuit. See U.S. Pat. No. 6,356,478. However, the cells are not of the same type and further, it is believed that such storage element is difficult to program/erase and verify.